Generating very large random numbers java

Question

How can we generate very large random number in java? I am talking something like 10000 digits? I know we have to use BigInteger but how can we do this? What is the most efficent way of doing something like this? Please provide a small example. Thank you.

score 22 · Accepted Answer · answered Nov 23 '11 at 15:31

22

Well, one way is to go to Random.org and download a one of the binary random files. The files are generated from atmospheric noise, so it's very random. I used it for Zobrist keys in my chess engine.

Alternatively you could go

BigInteger b = new BigInteger(256, new Random());

which will give you what you want. In this example, a BigInteger consisting of 256 bits.

answered Nov 23 '11 at 15:31

Jaco Van Niekerk

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+1. It was four constructors down in the documentation, I didn't see it :D. – Vlad Nov 23 '11 at 15:32

score 9 · Answer 2 · answered Nov 23 '11 at 15:27

9

Combine Random.nextBytes(byte[]) with BigInteger(byte[]).

import java.util.*;
import java.math.*;
class Test{
    public static void main(String[]_){

        int n = 16;

        Random r = new Random();
        byte[] b = new byte[n];
        r.nextBytes(b);
        BigInteger i = new BigInteger(b);

        System.out.println(i);
    }
}

answered Nov 23 '11 at 15:27

Vlad

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can you please explain how this works? i'm more or less confused on the bytes part.. thanks – Jeel Shah Nov 23 '11 at 15:35
You allocate an array of bytes, nextBytes fills it with random values, and then the BigInteger is initialized as the binary number defined by that sequence of bytes. But I think Jaco's solution is more concise. – Vlad Nov 23 '11 at 15:37

score -3 · Answer 3 · edited Feb 07 '19 at 17:02

-3

You can just type:

int number = (int)(Math.random() * 100);

Also, you can generate even bigger numbers if you change the multiplier:

int number = (int)(Math.random() * 1000);

P.S. You don't need to import a class.

edited Feb 07 '19 at 17:02

mburesh

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answered Feb 07 '19 at 16:12

Tefoo

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The question asks for numbers with 10000 digits. This answer does not even come close. – TT. Feb 07 '19 at 16:30
1000 digits won't fit into variable of int type. Multiplying random value by some constant makes it not random. – gudok Feb 07 '19 at 17:00
Although this answer might work for small numbers, the question is clear that it is for very large (much bigger than an int) numbers. So this is not a valid answer. – Stefan Feb 07 '19 at 19:11

Generating very large random numbers java

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